The present invention relates generally to lead assemblies for connecting implantable medical devices with selected body tissue to be stimulated by such devices, and more particularly to a stimulating electrode for use with such lead assemblies having low polarization which results in lower capture thresholds, increased sensing thresholds, and clearer evoked response signals.
Polarization, is an artifact that results from the accumulation of charge at the electrode/tissue interface post-stimulation. This after-potential prevents the accurate sensing of intrinsic cardiac electrical activity.
Current electrodes, both of the tip and ring variety, of modern pacing leads often employ a surface coating of titanium nitride (TiN). The implementation of TiN as a coating material of choice earlier provided a breakthrough in the pacing industry by exhibiting properties of corrosion resistance and providing an interface with increased electrode/tissue capacitance. The increase in interface capacitance is a result of the increased active surface area brought about by the fractal morphology of the sputter coated titanium nitride material. The increase in interface capacitance, in turn, lowers the polarization artifact typically seen following the pacing pulse. This falls out from the equation describing the after-potential or polarization given by equation (1), as follows.                                           U            C                    ⁡                      (                          t               greater than               T                        )                          =                              U            r                    (                      1            -                                          exp                ⁡                                  (                                                            -                      T                                                                                      R                        L                                            ⁢                                              C                        H                                                                              )                                            ⁢                              exp                ⁡                                  (                                                            -                                              (                                                  t                          -                          T                                                )                                                                                                            R                        L                                            ⁢                                              C                        H                                                                              )                                                                                        (        1        )            
where:
RL: resistance of the lead
CH: Helmholtz capacitance
Ur: charge voltage
Although the lowering of the polarization value can be achieved by increasing the lead length, which in turn increases RL, the resistance of the lead, it compromises the sensing functionality of the electrode and increases energy consumption. See Schaldach, xe2x80x9cThe Electrode-Electrolyte Interfacexe2x80x9d, Electrotherapy of the Heart, Spr-Vlg, 1992.
There are numerous disclosures of known leads and associated stimulating electrodes presented in the patented format. A number of the more pertinent known disclosures will now be discussed.
General disclosures of body implantable electrode constructions in which either or both of the leads and electrodes are constructed of titanium or titanium alloy are found in U.S. Pat. No. 5,181,526 to Yamasaki, U.S. Pat. No. 5,074,313 to Dahl et al. and U.S. Pat. No. 4,352,360 to King. In another instance as disclosed in U.S. Pat. No. 5,931,862 to Carson and U.S. Pat. No. 5,755,762 to Bush, a continuous sheath of open-celled porous plastic, preferably ePTFE, is used on the outside of a medical lead, extending along the lead body and the electrodes. Because the plastic is open-celled, when the pores are filled with saline, the lead can deliver electrical energy through the pores in the plastic. Pore size is chosen to discourage tissue ingrowth while allowing for defibrillation energy delivery and electrical signals through it. U.S. Pat. No. 5,824,016 to Ekwall discloses an implantable medical device for stimulating tissue including a pulse generator and associated electrode designed to prevent leakage currents in the output circuit. This is said to be accomplished by utilization of the electrode lead-to-tissue electrolytic interface capacitance (Helmholtz capacitance) with a highly leakage resistant layer interface together with and formed, for example, by titanium and titanium dioxide. U.S. Pat. No. 3,749,101 to Williamson discloses, without mention of, or concern for, polarization or Helmholtz capacitance, an electrode for muscle stimulation featuring a platinum electrode which has preferably been platinized to develop a coating of platinum black, contained in a second electrode housing of suitable electrode metal which is compatible with platinum such as titanium. Finally, the present invention recognizes that the use of a platinum black coating with materials other than titanium have long been known to reduce source impedance and polarization. See, for example, U.S. Pat. No. 5,628,778 to Kruse et al., col. 6, line 66 through col. 7, line 21. Indeed, the use of a platinum black coating even with titanium has long been known to reduce source impedance and polarization, witness U.S. Pat. No. 4,502,492 to Bornzin at col. 3, lines 57 through 65. Yet another noteworthy disclosure is presented in U.S. Pat. No. 5,851,896 to Summerfelt, which is concerned with providing a barrier layer for use in a high-dielectric constant material electrode. The barrier layer is defined as a conductive layer which minimizes diffusion of oxygen through itself down to the oxidizable layer, thus minimizing oxidation and degradation of the oxidizable layer. A preferred embodiment comprises an oxidizable layer (e.g. TiN), a conductive exotic-nitride barrier layer (e.g. Tixe2x80x94Alxe2x80x94N) overlying the oxidizable layer, an oxygen stable layer (e.g. platinum) overlying the exotic-nitride layer, and a high-dielectric-constant material layer (e.g. barium strontium titanate) overlying the oxygen stable layer.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.
The present invention, then, relates to an implantable stimulating electrode for directly contacting the endocardium of a human heart and exhibiting low polarization values (of less than about 0.3 mV). The stimulating electrode is intended for use with an implantable lead having an electrical connector coupled to the proximal end of a conductor for releasable attachment to a stimulating pulse generator. The electrode is comprised of a metallic substrate having an initially exposed outer surface substantially covered with a first inner layer of titanium nitride and a second outer layer of platinum black. The first inner layer of titanium nitride has a thickness of less than about 15 microns and the second outer layer of platinum black overlying the layer of titanium nitride, similarly, has a thickness of less than about 15 microns. The invention also includes a method of making the implantable stimulating electrode.
A more viable approach than increasing lead length, as earlier mentioned, and as presented herein, is to alleviate the polarization artifact by maximizing the Helmholtz capacitance, CH. The lowering of the after-potential or polarization artifact has a two-fold purpose:
(1) in conjunction with low-threshold leads, it decreases battery consumption; and
(2) it allows capture detection and, therefore, safer pacing at low battery consumption.
See, for example, de Voogt, W. G., xe2x80x9cPacemaker Leads: Performance and Progressxe2x80x9d, American Journal of Cardiology, March 1999, 83:5B, pages 187D-191D.
However, materials such as titanium have the propensity to oxidize readily in the atmosphere. For example, for a material such as titanium nitride, the ratio of final TiO2 thickness to initial TiN thickness is about 1.58. See U.S. Pat. No. 5,851,896 to Summerfelt. The thin insulating or semi-conducting oxide layer amounts to a capacitor in series with the Helmholtz capacitance (CH), thereby reducing the overall effective capacitance.
FIG. 1 depicts the equivalent circuit of an oxidizable interface as explained by Schaldach (xe2x80x9cThe Electrode-Electrolyte Interfacexe2x80x9d, Electrotherapy of the Heart, Spr-Vlg, 1992),
where:
RL: resistance of the lead
COX: capacitance associated with the oxide layer
ROX: resistance of the oxide layer
RF: Faradaic resistance
CH: Helmholtz capacitance                                           C            Eff                    =                                                    (                                  C                  OX                                )                            ⁢                              (                                  C                  H                                )                                                                    C                OX                            +                              C                H                                                    ⁢                  
                ⁢                              C            EFF                    =                                    C              H                        ⁡                          (                                                C                  OX                                                                      C                    OX                                    +                                      C                    H                                                              )                                                          (        2        )            
But: COX less than CH 
Therefore,       (                  C        OX                              C          OX                +                  C          H                      )     less than   1
It follows from (2), that:
CEFF less than CH
A primary feature, then, of the present invention is the provision of a stimulating electrode for use with an associated lead assembly having low polarization which results in lower capture thresholds, increased sensing thresholds, and clearer evoked response signals.
Another feature of the present invention is the provision of such a stimulating electrode exhibiting a polarization value of less than about 0.3 mV and for which the polarization value of an associated implantable lead is less than about one mV.
Still another feature of the present invention is the provision of such a stimulating electrode imparted with a layer of titanium nitride having a thickness of less than about 15 microns and with a layer of platinum black overlying the layer of titanium nitride having a thickness of less than about 15 microns.
Yet another feature of the present invention is the provision of such a stimulating electrode made by a method comprising the steps of, first, providing a blank metallic electrode having an outer surface substantially covered with a layer of titanium nitride, then immersing the blank electrode in an electroplating electrolyte solution comprised of chloroplatinic acid or an equivalent solution and then applying an emf across the volume of the electroplating electrolyte solution to thereby apply a coating of platinum black to the outer surface of the blank electrode resulting in a processed electrode.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate one of the embodiments of the invention, and together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.